Even with access to the vast fleets of starships and military materiel that the forges of Mars could churn out, the expansion outwards into the galaxy from Terra was slow, primarily due to a lack of the necessary manpower and skilled leadership. Without access to the Primarchs' unique genomes, the time required to create a Space Marine was far longer than ideal. The Emperor was also adamant that all the genetic material for new Astartesgene-seed must come directly from Him, to keep the gene-seed organs pure and 100% effective, as mutation had a habit of setting in even at this early date in the Astartes' existence.
Only a short time after he had launched the great expedition to reunite Mankind that he called the "Great Crusade," the Emperor was reunited with one of His Primarchs after His Great Crusade Expeditionary Fleet, the Pricipia Imperialis, reached the nearby Mining World of Cthonia during one of its first journeys through the Warp; this was Horus, the future Warmaster of the Imperial armies. Being the first Primarch rediscovered by the Emperor, the first of His beloved sons who had returned to Him, Horus and the Emperor soon formed a truly unique bond of friendship and paternal love, saving one anothers' lives on many occasions.
Horus was given command of the XVI Legion of Space Marines, the Luna Wolves, and he soon set the precedent for raising all future Astartes for his Legion from among the men of his former homeworld, in this case Cthonia. With the increased Space Marine production made possible by direct access to a Primarch's genome, the Great Crusade began picking up steam, with the Emperor's "Imperium of Man" bringing many new worlds into "Imperial Compliance" either through negotiation and diplomacy or outright conquest when all other methods failed.
Each world was also introduced to the Imperial Truth, the Emperor's chosen philosophy and the foundation for Imperial culture, which was stridently atheistic and rejected all forms of human religion or spirituality as mere superstition to be replaced by the cold rationalism of scientific progress. Though the Emperor loathed war, He understood that if Mankind was to survive the rigours of a hostile universe it must stand united, and that unity must be brought quickly, even if it had to be forced in certain instances.
After 30 Terran years of successful crusading, the Emperor discovered another of the Primarchs, the Wolf-King, Leman Russ on the frozen world of Fenris. Although Horus was pleased at the discovery of one of his brothers, he secretly hoped to always remain the Emperor's favourite son, no matter how many of his brothers were rediscovered.
Over time, all of the Primarchs were rediscovered by the various Expeditionary Fleets of the Great Crusade and given command of the Space Marine Legion for which their genetic code had been used as the foundation of that Legion's gene-seed. The order in which the Primarchs were discovered is not entirely clear in Imperial records. Horus was clearly the first to be rediscovered, and Alphariusof the Alpha Legion was the last.
Rogal Dorn of the Imperial Fists was the seventh of the Primarchs to be found, while Lion El'Jonsonwas discovered on the Death World of Calibanafter Leman Russ of the Space Wolves had already been recovered by the Emperor on Fenris.
Magnus the Red of the Thousand Sons was found on Prospero before his brother Lorgar of the Word Bearers welcomed the Emperor to Colchis. Angronof the World Eaters was rescued against his will from his homeworld of Nuceria after the Great Crusade recovered Perturabo of the Iron Warriorsfrom his homeworld of Olympia.
The Crusade also grew larger as the solar decades of conquest passed. With each new world brought into Imperial Compliance and made accepting of the Imperial Truth, its resources could be harnessed for the construction of ever more starships and the massive amounts of materiel needed to keep the entire enterprise moving forward. With the rediscovery of all of the Primarchs, the time required for Space Marine production was dramatically decreased; a new Astartes ready for combat could be created in as little as a standard year at the height of the Great Crusade.
This rapid creation process, however, would prove disastrous: mental defects and unique eccentricities began to creep into the ranks of the Astartes as the manpower needs of the Great Crusade called for larger and larger armies. Yet, by the early 31st Millennium, some 200 standard years after it had begun, the Great Crusade had reunited vast swathes of the galaxy under the leadership of the Emperor of Mankind, seemingly initiating a new Golden Age of progress and reason for the human race.